Preheat the oven to 375°F. Cut the tops off each bell pepper and remove the seeds and membranes. Use a small knife to carve simple jack-o'-lantern faces into the sides of the peppers.
Heat the olive oil in a large skillet over medium heat. Add the diced onion and minced garlic and cook for 2 to 3 minutes until the onion softens.
Add the ground beef to the skillet and cook, breaking it up with a spoon, until it is browned and no longer pink. Drain any excess fat.
Stir the cooked rice, tomato sauce, salt, and black pepper into the beef mixture. Cook for another 2 minutes until everything is heated through and well combined.
Place the carved peppers in a baking dish. Spoon the beef and rice filling evenly into each pepper. Sprinkle the shredded cheddar cheese over the top of the filling.
Bake for 25 to 30 minutes until the peppers are tender and the cheese is melted and lightly browned.